Communications networks which employ ATM (asynchronous transfer mode) techniques are well known. In general, the data elements travel at high bit rates through a network in the form of cells. Each cell has a header field and an information field or payload. The header field is used for the multiplexing or routing of the associated cell through different element of the network (for instance terminals, multiplexers, scramblers, switches and the like). The information field typically comprises useful data destined for a user of the network to whom the cell is addressed.
The two fields mentioned above have a fixed format. The leader field is typically comprised of 5 data bytes and the information field is typically comprised of 48 data bytes Among the 5 data bytes of the header field, there can be distinguished two fields used to identify multiplexing or routing functions namely 8 bits for a virtual path identifier (or VPI) and 16 bits for a virtual channel identifier (or VCI).
In general, transmission techniques or devices which work at relatively high bit rates are available for deployment in an ATM network. Such techniques or devices are available especially for transmission between intermediate nodes within a given network. In contrast, at the edges or boundaries of a backbone network, problems are often encountered in ensuring data distribution on to end users at sufficiently acceptable bit rates. Indeed, the link between the backbone network and a final subscriber is often provided by a telephone line, such that it may be difficult to achieve bit rates of the order of several Mbits/s.
For end links of the kind discussed above which comes to a backbone network, it is possible to utilize DSL modulation, or a derivative modulation such as ADSL, HDSL, SDSL or the like, in order to provide for communications at rates of up to several Mbits/s. However, this may be insufficient for supporting some types of services. It must be noted that apart from the main portion of the bandwidth that is used to convey useful information from the information field or payload of cells, an additional portion of the bandwidth is used to convey the header field of cells. Furthermore, an additional part of the bandwidth must also be reserved for network administration or management functions. For instance, such functions may pertain to the management of the various network elements or devices to which cells are addressed.
Therefore, based on the foregoing, another approach to the optimization of bandwidth would be desirable. McTiffin has proposed a solution to this problem in a special case of RF transmission between an ATM network and a subscriber. This known technique, for instance as described in the U.S. Pat. No. 5,406,550, proposes the elimination of the VPI end VCI identification fields and the use of a radio coding that identifies the addressee. However, this approach may have several drawbacks. First of all, an approach of this kind is intended for radio link transmissions in which the transmitted signals bear an identification of the addressee by way of radio coding. This identification is not readily adaptable to the case of wire links. Adding an information element to a wire link according to the teachings of McTiffin would require an increase in the processing capacity of network terminal equipment, such as a network termination unit (NTU). However, an increase in processing capacity of this kind would be unnecessary since is the case of a wire link, there is typically only one link between the NTU and the backbone network.
Moreover, the McTiffin technique is better suited to CDMA (Coded Division Multiple Access) or TDMA (Time Division Multiple Access) techniques which make use of cell subdivision to encapsulate an integer number of ATM cells within a radio frame. An encapsulation of this kind requires additional bandwidth for the introduction of intervals of silence between two cells intended for one and the same addressee. The approach according to McTiffin is therefore compatible solely with radio transmission systems that delineate cells in order to separate them and therefore requires technique or equipment that are specifically adapted for executing this approach. Lastly, this known approach is not compatible with the transmission of a continuous flow of cells, since their synchronization is provided by the transmission network.
It is therefore generally an object of the present invention to attempt to overcome or alleviate the various drawbacks of the prior art which were previously mentioned. More specifically, it is one of the objects of the present invention to attempt to provide a method for the transmission of data elements organized into cells (for example ATM type cells) that can be used to derive a bandwidth gain on a portion of the transmission, for instance at the ends of a transmission network. An additional object of the invention is to attempt to provide a method of this kind which may make it possible to provide additional services beyond that of the simple transmission of cells, for instance based on a standard telephone type terminal link. Examples of these additional services include those relating to leased lines, Frame Relay transmission, Internet services and other services that will be apparent to those skilled in this art.
It is another object of the invention to attempt to provide a method of the kind described above that is independent of the transmission system implemented at the level of the subscriber. In particular, an object of the invention is to seek to provide a method of this kind that could be adapted to any type of appropriate network (RF, wire, or the like).
It is a further object of the invention to seek to provide a method of the kind described above that can be used to simplify the administration of the network or at least not render it more complex. It is also an object of the invention to attempt to provide a communications system as well as a network termination unit or other apparatus for use with such a system, all of which implement a method along the lines previously described.
These objects as well as others shall become apparent from the detailed description of the present invention which follows.